The present invention generally pertains to molding systems and methods and is particularly directed to control of hold pressure and clamping in a stacked multi-parting molding system having desynchronized injection periods.
A stacked multi-parting molding system includes at least three molding blocks disposed for movement along a common axis with respect to each other. Molding cavities are defined between adjacent molding blocks. In a desynchronized stacked multi-parting molding system that includes left, center, and right molding blocks for defining a left molding cavity between the left and center molding blocks and a right molding cavity between the center and right molding blocks, positioning means move the left, center and right molding blocks along the common axis with respect to each other in such a sequence that the opening and closing of the left and right molding cavities may be desynchronized. Desynchronized stacked multi-parting molding systems are described in U.S. Pat. Nos. 4,400,431, 4,464,327 and 4,539,171, all to Jens Ole Sorensen and 4,146,600 to Elly et al. Desynchronized injection of plastic material into the molding cavities may take place when both mold cavities are closed, as well as when one molding cavity is open and the other mold cavity is closed.
Hold pressure is the pressure applied to plastic material injected into a molding cavity of an injection molding system after the molding cavity is initially filled with plastic by an injection system. Hold pressure is applied to force additional plastic material into the molding cavity as the initially injected material shrinks in the molding cavity upon hardening in order to assure that the plastic material completely fills the cavity. Otherwise the molded product may be crimped.
In a prior art stacked multi-parting molding system, the injection system is used to apply hold pressure in the molding cavities. However, the use of the injection system to apply hold pressure may increase the cycle time because less time will be available during the molding cycle for plasticization of the molding material prior to injection. Also, with typical desynchronized stacked molding systems, when at least one of the molding cavities is opened, the injection system is separated from the remainder of the feed system that carries the plastic material to the molding cavities, thereby reducing the time available for the injection system to apply hold pressure.
While hold pressure is being applied to the molding cavities, the molding cavities must be tightly clamped. While both molding cavities are closed, the means that position the molding blocks apply an axial clamping force to the molding cavities along their common axis to thereby tightly clamp the molding cavities. However, whenever one molding cavity is opened, this axial clamping force is no longer applied by the positioning means. Although Elly et al describe the use of wedge-shaped locking means for maintaining one molding cavity in a closed position while the other molding cavity is in an open position, the axial clamping force that is applied by the wedge-shaped locking means to the molding cavity that remains closed is significantly less than that applied by the positioning means when both molding cavities are closed, and typically is insufficient to enable adequate hold pressure to be applied to the closed molding cavity.